The present invention relates to the general field of core exhaust mixers, having a variable area, for turbo-fan jet engines with low by-pass ratio for supersonic aircraft.
A turbo-fan jet engine with low by-pass ratio for supersonic aircraft is composed primarily of a dual-flow gas generator (cold flow and hot flow) prolonged by a nozzle, which defines a gas ejection passage.
Turbo-fan jet engines with low by-pass ratio equipping supersonic aircraft, which are intended for civil aviation, must meet two requirements: on the one hand, they must have the lowest possible drag in flight during transonic and supersonic cruising, and on the other hand, they must have an acceptable noise level during takeoff of the aircraft, the certification authorities being increasingly demanding as regards sound emissions from the jet engines of civil aircraft.
However, these two requirements are contradictory. In fact, the first requirement leads to jet engine architectures of small diameter, while the second requirement requires the thrust to be increased by augmenting the gas output, which leads to large fan (and thus jet engine) diameters.
A known solution meeting these requirements is the use of a core exhaust mixer having a variable area. A mixer of this type makes it possible, during the takeoff phases of the aircraft, to introduce into the jet engine an air flow, external to the jet engine, in order to mix it with the gas flow coming from the gas generator. In fact, mixing external air with the gas flow coming from the gas generator makes it possible to augment the gas output produced by the jet engine. Also, at constant thrust, the ejection speed of the gases can be reduced, compared to a turbo-fan jet engine without a mixer. As the jet noise increases with the ejection speed of the gases, reduction in this speed generates a significant drop in noise level during takeoff.
In practice, external air is introduced into the jet engine downstream of the gas generator using apertures distributed over the entire circumference of the nozzle. The air introduced in this way mixes with the gas flow from the gas generator by means of guides which extend radially through the ejection passage of the gas flow. These guides are movable between a position in which they unblock the apertures and allow mixing (during the takeoff phases of the aircraft) and another position in which they block the apertures during the other flight phases.
Although satisfactory, this solution is disadvantageous in that significant lengthening of the jet engine is required in order to assist mixing of the external air flow and the gas flow from the gas generator. However, the jet engine is lengthened to the detriment of its size. Moreover, it is generally necessary to use an acoustic lining on the internal wall of the exhaust nozzle of the jet engine, in order to absorb the most harmful sound frequencies.